In so-called brushless direct-current (BLDC) motors, motor rotation is caused by electronic switching or “commutation” of drive current among a plurality of motor windings. The commutation induces a rotating component to the overall magnetic field associated with the windings, and the interaction of this rotating magnetic field component with the rotor's permanent magnets causes rotation of the rotor. In one example, one common BLDC configuration employs three windings offset from each other by 120 degrees of electrical phase.
Commutation can be performed in many different ways. A set of motor position sensors such as Hall-effect magnetic sensors are arranged about the motor rotational axis with a precise spacing. As the rotor rotates, the permanent magnets rotate, causing the output of each Hall-effect sensor switches in a binary fashion. The collective outputs of the sensors are used to control commutation. In one simple arrangement, three sensors offset from each other by nominally 120 electrical degrees are used. Each of six distinct sets of the three binary outputs of the sensors maps directly to a corresponding commutation state in which a particular pair of the motor windings is driven with current with a particular polarity. Thus commutation occurs precisely at the time that the binary output from one of the sensors switches from one binary state to the other. Ideally, the sensors are located such that this transition occurs exactly midway between two successive peaks of a torque characteristic of the motor, so that the value of the torque at this “valley” location is as high as possible. This ideal configuration provides the highest minimum torque value which may be especially needed to maintain proper operation under high load conditions.
U.S. Pat. No. 6,239,564 of Boe et al. discloses a motor controller that senses an impending motor stall accompanied by magnetic saturation of the energized windings and advances the commutation state to the next succeeding state in advance of the corresponding transition of the Hall-effect sensors. Boe et al. describe this operation as increasing the torque to the motor to enable it to stay out of the stall condition.